A reduction gear transmission that reduces the rotation speed of an output shaft of a motor and outputs the reduced rotation speed is known to the art. In order to realize a large reduction ratio in this reduction gear transmission, a plurality of gears is combined. However, when too many gears are combined, the reduction gear transmission increases in size, and the configuration of the reduction gear transmission becomes more complex. A large reduction ratio may be realized by unitizing the reduction gear transmissions and connecting the unitized reduction gear transmissions in series.
Japanese Patent Application Publication No. 2001-193801 sets forth a reduction gear transmission having a first reduction gear unit (a first stage gear device) and a second reduction gear unit (a second stage gear device). The first reduction gear unit is an inner meshing planetary gear reduction device. The inner meshing planetary gear reduction device comprises an eccentric body shaft, an external gear, and an internal gear. The eccentric body shaft rotates coaxially with an output shaft of a motor (a motor shaft) and comprises an eccentric cam. The external gear engages with the eccentric cam. The internal gear allows the external gear to rotate eccentrically while maintaining a meshed state with the external gear. Further, the internal gear surrounds the external gear. The internal gear has a number of teeth differing from the number of teeth of the external gear. The external gear rotates with respect to the internal gear while rotating eccentrically around an axis line of the internal gear. In the present specification, the phenomenon of the external gear rotating eccentrically around the axis line of the internal gear is also termed revolving orbitally around the axis line of the internal gear. Further, the case where “the external gear rotates”, refers to the external gear rotating relative to the internal gear. An output shaft (a slow speed shaft) is coupled with the external gear. Torque of the first reduction gear unit is transmitted to the second reduction gear unit via the output shaft. The second reduction gear unit reduces the speed of an inputted rotation via two shafts that are disposed parallel to one another. That is, in the second reduction gear unit, an output shaft having a through hole formed in its center is connected to an input shaft that rotates integrally with the output shaft of the first reduction gear unit by a helical pinion and a helical gear, and the number of rotation of the output shaft of the second reduction gear unit is reduced to less than the number of rotations of the input shaft of the second reduction gear unit. In the technique of Japanese Patent Application Publication No. 2001-193801, a large reduction ratio is realized by connecting the first reduction gear unit and the second reduction gear unit in series. A large torque can be obtained by realizing the large reduction ratio.